=over

=item $PROGRAM_NAME

=item $0
X<$0> X<$PROGRAM_NAME>

Contains the name of the program being executed.

On some (but not all) operating systems assigning to C<$0> modifies
the argument area that the C<ps> program sees.  On some platforms you
may have to use special C<ps> options or a different C<ps> to see the
changes.  Modifying the C<$0> is more useful as a way of indicating the
current program state than it is for hiding the program you're
running.

Note that there are platform-specific limitations on the maximum
length of C<$0>.  In the most extreme case it may be limited to the
space occupied by the original C<$0>.

In some platforms there may be arbitrary amount of padding, for
example space characters, after the modified name as shown by C<ps>.
In some platforms this padding may extend all the way to the original
length of the argument area, no matter what you do (this is the case
for example with Linux 2.2).

Note for BSD users: setting C<$0> does not completely remove "perl"
from the ps(1) output.  For example, setting C<$0> to C<"foobar"> may
result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
and the " (perl)" suffix are shown depends on your exact BSD variant
and version).  This is an operating system feature, Perl cannot help it.

In multithreaded scripts Perl coordinates the threads so that any
thread may modify its copy of the C<$0> and the change becomes visible
to ps(1) (assuming the operating system plays along).  Note that
the view of C<$0> the other threads have will not change since they
have their own copies of it.

If the program has been given to perl via the switches C<-e> or C<-E>,
C<$0> will contain the string C<"-e">.

On Linux as of perl v5.14.0 the legacy process name will be set with
C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
perl has done since version 4.000.  Now system utilities that read the
legacy process name such as ps, top and killall will recognize the
name you set when assigning to C<$0>.  The string you supply will be
cut off at 16 bytes, this is a limitation imposed by Linux.

Mnemonic: same as B<sh> and B<ksh>.

=back